Aluminum-titanium alloy with high specular reflectivity, reflecting coatings comprising same and mirrors and parts comprising said coating

ABSTRACT

The metal alloy based on aluminium and titanium includes an aluminium content between 80 and 90 atomic percent and a titanium content between 10 and 20 atomic percent. The alloy is microcrystalline and outside thermodynamic equilibrium, the alloy is thereby resistant to oxidation and corrosion and has at the same time remarkable adhesion to polymer materials. The metal alloy can be in the form of a reflecting coating of a thickness ranging between 0.01 and 3 μm, covered with a protective film of a polymer material. A mirror having specular reflectivity not less than about 65%, good resistance to corrosion and oxidation includes a substrate of a polymer material supporting the reflecting coating.

FIELD OF THE INVENTION

The present invention relates to a new aluminium-titanium alloy of highspecular reflectivity, reflective coatings comprising the alloy andmirrors and components including the coating.

The invention relates more particularly to the design and manufacture ofspecial mirrors. Special mirrors are mirrors whose opticalcharacteristics must lie within a well-defined range and which must bepreserved in their entirety, even if they are subjected to severeconstraints. Examples of severe constraints that are not limiting on theinvention are particularly corrosive atmospheres, such as moistatmospheres, with or without condensation, in the presence of chemicalagents such as sodium chloride, for example. Even more particularly, butwithout this being limiting on the invention, the invention relates tomirrors made by depositing a layer of metal onto a plastics material orcomposite material support; in the latter case, the interface betweenthe reflective coating and the substrate, which is usually a polymer, isparticularly sensitive to the environment and is the site ofirreversible deterioration.

BACKGROUND OF THE INVENTION

Special mirrors to resist aggressive environments are often made bydepositing a layer of metal onto a support providing an adequatestructure. The metal layer is then protected against mechanical,chemical or other aggression by a transparent protective film. Onenon-limiting example of a protective film is a metal oxide oroxynitride, although for economic reasons it is very often a polymer.

A mirror of this kind therefore includes two interfaces, asubstrate/reflective coating interface and a reflectivecoating/protective film interface; these are weak points because theycan be the site of separation leading to tarnishing, blistering and, inall cases, serious deterioration of the mirror.

In the prior art, the special mirrors described above are generally madeby depositing a metal referred to as “from the platinum mine”, i.e. fromgroup VIII of the periodic table of the elements, and more particularlyrhodium, ruthenium or palladium. These metals are of acceptablereflectivity, close to 60%. They are not sensitive to tarnishing andhave good resistance to mechanical aggression because of their highhardness. However, this technique has major disadvantages connected withthe difficulty of making coatings with these metals and their very highcost.

Some manufacturers make mirrors by depositing pure aluminium or silverprotected by a transparent polymer onto a metal, polymer or ceramicsubstrate. Such mirrors have excellent optical properties but cannot beused in an aggressive atmosphere as they deteriorate very quicklythrough tarnishing, blistering and scaling of the coating.

A layer of titanium has been used, associated or not with a layer ofchromium, as described in FR 2768096, but these structures deterioratequickly in an aggressive environment, to the point that FR 2768096recommends depositing the layers onto the rear face of a transparentsupport, indicating that it is then particularly well protected frommechanical and chemical influences.

Alloys of titanium and aluminium are known in the art. They are used toconfer mechanical properties on the titanium; these alloys have a lowaluminium content and generally contain copper.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a metal alloysimultaneously having the properties of being a good reflector and offorming very strong chemical bonds with the substrates, in particularpolymer substrates, so that the two interfaces mentioned above areresistant to most forms of atmospheric aggression, whether of anindustrial or saline nature.

Another object of the invention is to provide metal alloys that areintrinsically resistant to corrosion and therefore to tarnishing.

A further object of the invention is to provide an alloy formingchemical bonds which are not degraded by variations in temperature overa wide range of use with the substrates, in particular with polymersubstrates.

It is also an object of the invention to provide reflective coatings atreduced cost.

The above objects, together with others that will become apparent onreading the following description, are satisfied by the presentinvention which provides a microcrystalline metal alloy based onaluminium and titanium whose aluminium content is from 80 to 90 atomicpercent, whose titanium content is from 10 to 20 atomic percent, that isnot in thermodynamic equilibrium, and that is therefore resistant tooxidation and to corrosion, and that simultaneously has a remarkablecapacity for adhesion to polymer materials.

In the context of the invention, the term “microcrystalline” refers to acrystal having a size less than approximately 1 μm. According to theinvention, a microcrystalline alloy is therefore constituted of crystalshaving a size less than approximately 1 μm.

The performance of the metal alloy of the invention, in particular withregard to its reflectivity, is improved if the alloy is purer. However,up to 2 atomic percent of impurities can be tolerated withoutsignificantly affecting the performance of the alloy.

DETAILED DESCRIPTION OF THE INVENTION

Consequently, in one preferred embodiment of the invention the metalalloy comprises not more than approximately 2 atomic percent ofimpurities.

In a preferred embodiment of the invention the metal alloy has analuminium content from 84 to 87 atomic percent and a a titanium contentfrom 13 to 16 atomic percent.

It has been necessary to overcome a major prejudice to arrive at thepresent invention because it is well known in the art that addingaddition elements to aluminium will normally significantly reduce itsreflectivity.

Alloys of titanium and aluminium are known in the art but these aregenerally alloys with a low content of aluminium and generally containcopper and are used to confer mechanical properties on the titanium.

Aluminium alloys with a low content of titanium have been described,generally associated with equivalent quantities of manganese andmagnesium, the titanium content not exceeding one atom percent. Intendedfor the fabrication of hollow bodies under pressure, such alloys aretotally outside the field of application and the scope of the invention.

The publication “Binary Alloy Phase Diagrams” by T. B. Massalski, vol 1,pages 175-176, discloses alloys of aluminium and titanium whosealuminium content is from 80 to 90 atomic percent and whose titaniumcontent is from 10 to 20 atomic percent. These alloys, which are outsidethe scope of the present invention, are in thermodynamic equilibrium andare not microcrystalline. They do not have the required resistance tooxidation and to corrosion and do not have a significant capacity foradhesion to polymer materials.

The present invention also provides a reflective coating consisting of alayer of metal alloy according to the invention described above coveredwith a protective film.

The metal alloy is an alloy based on aluminium and microcrystallinetitanium which is not in thermodynamic equilibrium, whose aluminiumcontent is from 80 to 90 atomic percent and whose titanium content isfrom 10 to 20 atomic percent.

In a preferred embodiment of the invention the reflective coatingconsists of a layer of metal alloy covered with a transparent protectivefilm, and the metal alloy is an alloy based on aluminium and titaniumwhose aluminium content is from 80 to 90 atomic percent, whose titaniumcontent is from 10 to 20 atomic percent and whose impurities content isat most equal to approximately 2 atomic percent.

In the reflective coatings of the present invention the protective filmis usually a polymer material.

The thickness of the metal alloy layer is generally from 0.01 to 5 μm.

Below 0.01 μm the coating is quasi-transparent. Beyond 5 μm thereflectivity is degraded, although the other properties are notaffected.

The thickness of the layer of metal alloy is preferably from 0.01 to 3μm and more preferably from 0.01 to 0.5 μm.

The present invention also provides a mirror comprising a substratesupporting a reflective coating as described above.

The substrate is generally a polymer material.

To obtain a special mirror having a specular reflectivity at least equalto approximately 65%, good resistance to corrosion and to oxidation, itis necessary to choose a reflective coating consisting of a layer ofmetal alloy covered with a transparent protective film, characterized inthat the metal alloy is an alloy that is based on aluminium andmicrocrystalline titanium, that is not in thermodynamic equilibrium,whose aluminium content is from 80 to 90 atomic percent, whose titaniumcontent is from 10 to 20 atomic percent, and whose impurities content isat most equal to approximately 2 atomic percent, and in which thethickness of the alloy layer is from 0.01 to 3 μm.

When the reflective coatings of the invention are deposited on a polymersubstrate and protected by a transparent protective film, thecombination can be exposed to a corrosive atmosphere such as that of thesalt mist test of ISO standard 9227 without affecting the reflectivityor causing defects to appear at the interfaces.

The aluminium/titanium alloy according to the invention can be depositedby any appropriate means; however, vacuum deposition techniques such ascathode sputtering in its various forms, evaporation and co-evaporationunder certain conditions (which can readily be determined by the skilledperson in order to obtain microcrystals) represent means that areparticularly indicated for making such materials in the form of thinlayers.

The mirrors of the invention, in particular the special mirrors, can beparts of components such as automobile rear view mirrors or billboards.

The alloy in accordance with the invention has a very high affinity forand a very high adhesion to polymers.

The corrosion resistance is also high because the reflecting face of themirror is in direct contact with the atmospheric agents and is notprotected from oxidation.

Nippon Jidosha patent JP 61133902 describes a mirror consisting of aglass substrate on which are successively deposited a transparent layerof TiO₂ and a reflective layer of an aluminium alloy, for examplealuminium titanium alloy. The aim is to make a coloured mirror, thecolour being produced by luminous interference within the TiO₂ layer.The titanium dioxide is the most important component of the colouredmirror. Note that the reflecting layer of aluminium alloy is on the rearpart of the glass relative to the light rays; its reflecting face istherefore protected from oxidation. What is more, the aluminium alloy ofthe above patent, which is in contact with a ceramic, unlike that of thepresent invention, does not have to adhere to or have an affinity for apolymer, ceramics being entirely different from polymers on thephysical-chemical plane.

The invention will now be described in more detail with reference to thefollowing non-limiting examples.

EXAMPLE 1

This example concerns the production of a mirror to be used outdoors.

The structure of the mirror was made of polycarbonate (substrate).

It had to resist abrasive wear by particles entrained by the wind andalso corrosion due to rain and industrial atmospheres. To this end, a0.1 μm thick layer of aluminium titanium alloy according to theinvention containing 85 atomic percent aluminium, the remainder beingtitanium, was deposited by vacuum deposition, for example cathodesputtering, and then protected by a transparent coating (protectivefilm) that was resistant to wear and was based on polysiloxane.

The reflectivity of the mirror was 67%±1%, the adhesion of its coatingwas measured in accordance with ISO standard 2409 and the result wasclass 0. The mirror was aged artificially by exposure to a salineatmosphere according to ISO standard 9227 for 400 hours. It was thenwashed, dried and tested again for adhesion; the result was class 0, nounsticking or scaling having appeared. The reflectivity after the testwas 66%±1%, the difference relative to the initial value was in theorder of magnitude of the measurement errors.

By way of comparison, a mirror made conventionally by depositing purealuminium of the same thickness onto a polycarbonate substrate and thencovering it with the same polysiloxane coating was subjected to the sametest; after 400 hours exposure to salt mist, the coatings had comeunstuck over approximately 30% of the surface of the mirror, which wasunusable.

EXAMPLE 2

A mirror for use in infrared heating was produced. The mirror had to beable to support working temperatures of 200° C. without affecting itsperformance. To this end, a polyetherimide structure was used and wascoated by vacuum deposition with a layer of aluminium titanium alloyaccording to the invention. Its composition was 87 atomic percentaluminium and 13 atomic percent titanium. Its thickness was 5micrometers.

The intrinsic reflectivity of the mirror was 70° C. The adhesion of thecoating, tested by the method cited in example 1, gave a result of class0. The mirror was subjected to a series of tests alternating periods ofone hour at 200° C. and two hours at room temperature. The test wasstopped after ten cycles and the characteristics were measured again.The adhesion test result was class 0, the reflectivity was unchanged.

By way of comparison, the same tests were carried out on two prior artmirrors, one obtained by coating a polyetherimide substrate with 1micrometer of gold and the other by coating the same substrate with purealuminium. After ten cycles, the gold coating had become partly unstuck,rendering the mirror unusable, the reflectivity of the aluminiumcoating, that was 95%, was reduced to 52% by the growth of a surfaceoxide layer.

EXAMPLE 3

A billboard was made intended to be displayed outdoors. The billboardwas made from acrylonitrile-butadiene-styrene (ABS) and had to have abright metallic appearance. The service life on site had to be greaterthan five years.

To this end, a billboard was metal-plated with an alloy of aluminium andtitanium according to the invention containing 82 atomic percentaluminium and 18 atomic percent titanium. Its thickness was 0.3micrometers. It was then coated with a transparent protective varnish 20micrometers thick. The adhesion of the coatings measured by the testcited in example 1 was class 0; its reflectivity was 68%.

Ageing was simulated by 100 hours exposure to salt mist in accordancewith the standardized test cited in Example 1. At the end of the test,the billboard was washed, dried and its characteristics were measuredagain. The adhesion remained unchanged and the reflectivity was 65%.This slight variation was associated with slight surface attack of theprotective varnish by salt mist.

By way of comparison, the same tests were applied to a billboard coatedwith a 0.1 micrometer thick deposit of chromium by the usualelectroplating technique and protected by the same varnish. Thereflectivity was 70% and the adhesion class was 0. After exposure tosalt mist the reflectivity was reduced to 66% for the same reasons butthe adhesion class was only 1.

EXAMPLE 4 Non-microcrystalline Alloy (not in Accordance with theInvention)

To make high-performance mirrors on a light plastics material substrate,an ingot of AlTi alloy containing 13% titanium was cast and rolled toobtain a film 10 μm thick. X-ray examination of the film confirmed thatits crystallographic structure conformed to that predicted by thealuminium titanium state diagram.

The alloy was not microcrystalline and was in thermodynamic equilibrium.

The alloy film was then rolled conjointly with a polymer film to reducethe thickness of the metal layer to 0.2 μm.

The metallized plastics material film was then glued to thepolycarbonate substrate constituting the framework of the mirror. Thewhole was then covered with a layer of transparent antiscratch varnishto protect the reflecting surface from damage, this technique enablingeasy industrialization and low cost mass production.

The reflectivity of the mirror was 75%.

To verify its durability, the mirror was subjected to the salt mist testdescribed in Example 1 above. After testing for 24 hours, which wasnotably too short, the layer of protective varnish was particularlydegraded. Examination with a microscope showed that the origin of thedegradation was insufficient adhesion of the polymer varnish to thealuminium titanium alloy obtained in this way.

What is claimed is:
 1. A reflective coating adapted to be submitted tooxidation and corrosion and adapted to be adhered to a polymer material,comprising a layer of metal alloy deposited under vacuum, said metalalloy containing from 80 to 87 atomic percent of aluminium and 13 to 20atomic percent of titanium.
 2. The reflective coating of claim 1,wherein the metal alloy has a grain size of at most 1 micron.
 3. Thereflective coating of claim 2, wherein the metal alloy is not inthermodynamic equilibrium.
 4. The reflective coating of claim 1, whereinthe metal alloy contains not more than about 2 atomic percent ofimpurities.
 5. The reflective coating of claim 1, wherein the aluminiumcontent is from 84 to 87 atomic percent and the titanium content is from13 to 16 atomic percent.
 6. The reflective coating of claim 1, furthercomprising a protective film on the layer of the metal alloy.
 7. Thereflective coating of claim 6, wherein the protective film is atransparent protective film.
 8. The reflective coating of claim 6,wherein the protective film comprises a film of polymer material.
 9. Thereflective coating of claim 1, wherein the metal alloy layer is from0.01 to 5 microns thick.
 10. The reflective coating of claim 9, whereinthe metal alloy layer is from 0.01 to 3 microns thick.
 11. Thereflective coating of claim 10, wherein the metal alloy layer is from0.01 to 0.05 microns thick.
 12. A mirror comprising a substrate and areflective coating adapted to be submitted to oxidation and corrosion,comprising a layer of metal alloy deposited under vacuum on thesubstrate, said metal alloy containing from 80 to 90 atomic percent ofaluminium and 10 to 20 atomic percent of titanium.
 13. The mirror ofclaim 12, wherein the substrate comprises a polymer material.
 14. Themirror of claim 12, wherein the metal alloy layer is covered with aprotective film.
 15. The mirror of claim 12, wherein the titaniumcontent is not less than 13 atomic percent.
 16. The mirror of claim 15,wherein the titanium content is not more than 16 atomic percent.
 17. Themirror of claim 12, having a specular reflectivity at least equal toapproximately 65% and good resistance to corrosion and to oxidation. 18.A component comprising the mirror according to claim
 12. 19. Thecomponent of claim 18, constituting an automobile rear view mirror. 20.The component of claim 19, constituting a billboard.
 21. A mirrorcomprising a substrate supporting a metal alloy based on aluminium andtitanium whose aluminium content is from 80 to 90 atomic percent and thetitanium content is from 10 to 20 atomic percent, the grain size of thealloy being not more than 1 micron and the alloy not being inthermodynamic equilibrium, and a protective film covering the metalalloy.
 22. The mirror according to claim 21, wherein the substratecomprises a polymer material.
 23. The mirror according to claim 21,wherein the titanium content is not less than about 13 atomic percent.